AN EPIDEMIOLOGIC STUDY OF VENEZUELAN EQUINE ENCEPHALOMYELITIS IN COSTA RICA, 1970

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Abstract
Martin, D. H., G. A. Eddy, W. D. Sudia, W. C. Reeves, V. F. Newhouse and K. M. Johnson (MARU, Box 2011, Balboa Heights, Canal Zone). An epidemiologic study of Venezuelan equine encephalomyelitis in Costa Rica, 1970. Am J Epidemiol 95: 565–578, 1972.—An epidemic of Venezuelan equine encephalomyelitis (VEE) occurred in Guanacaste Province, Costa Rica, in August 1970, shortly after completion of a baseline serologic survey of equines and humans and a study of vector populations. Equines in the northern part of the province where the disease initially appeared were shown to be largely susceptible to VEE prior to the outbreak, whereas those in the south had a high prevalence of antibody to an endemic strain of virus. The disease spread southward along the narrow Pacific coastal strip and penetrated 14 km inland in the upper Tempisque River Valley. Differences in ecologic habitat reflected by mosquito populations, high levels of immunity to endemic virus in the southern part of the province, and a prompt vaccination program with the TC-83 vaccine were important factors in limiting the extent of the outbreak. In the village of Cuajiniquil, 64% of infected equines died and mortality appeared not to be influenced by age or sex. Encephalitic signs were associated with appearance of virus-specific neutralizing antibodies and disappearance of detectable viremia. Equine deaths ceased abruptly eight to nine days after vaccination. Twelve per cent of humans in Cuajiniquil were infected by VEE during the outbreak, with highest rates among males and adults. The initial human case was observed approximately two weeks after the first equine death. Only one human infection was detected during a three-month interval beginning one month after all surviving equines were vaccinated. No human deaths attributed to VEE were observed. VEE virus was isolated from 20 equines and four humans, and from 12 Deinocerites pseudes D. & K., and four Aedes taeniorhynchus (Wied.) mosquito pools made from CDC light trap collections. Blood source in 70 of 74 engorged D. pseudes was identified: 34% equine, 33% bovine, 16% avian, 14% porcine, and 3% human. Selected human, equine and mosquito virus isolates were tested by a short incubation hemagglutination-inhibition technique and found to be homologous to the 1969 Guatemala I-B epidemic strain of VEE.

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